The invention relates to electric-discharge heating systems and, more particularly, the invention relates to controlled power supplies.
The invention can be used in electric arc furnaces, in welding equipment, as well as in other similar devices, in which use is made of a short-circuit operating mode.
Known in the art are two ways of building up the electric circuit of electric arc furnaces. According to one way, the supply source is based on an uncontrolled or controlled reactor connected in series with a primary winding of a control transformer. The secondary winding of this transformer is connected to a diode rectifier which, in turn, is connected to the furnace electrodes.
The systems based on a diode rectifier with a non-controlled reactor has a low power factor and causes significant voltage fluctuation in the supply network.
The electrode position regulators are designed for maintaining an electric arc of the same length, and a large number of transformer steps are used for controlling the power during the process.
In order to eliminate the negative effect on the other consumers in networks of low and medium power range, dynamic compensators of reactive power are used. This increases the weight and overall dimensions of the power supply as a whole.
Known in the art is an ac-to-dc inverter with an uncontrolled rectifier connected in a bridge circuit comprising a matching transformer whose secondary winding has a tap connected directly to one input of the rectifier bridge, while the end and beginning of the secondary winding are connected to the remaining inputs of the rectifier bridge through a capacitor and a choke respectively. This inverter was proposed for supply of an electric arc and makes it possible to enhance the power factor and reduce the supply network voltage fluctuation. However, the use of this inverter with the known devices to control the position of the electrode for arc sources, in which there is no controlled elements (for example, for electric arc furnaces with a diode rectifier or for furnaces operating with alternating current) needs a switch and a transformer with a large number of control steps. This reduces the operational reliability of the power supply, deteriorates its weight and size and, while continuous control of output power, which is very important for efficient run of the process, is not provided.
Furthermore, higher current harmonics appearing during the rectification do not allow this inverter to be used in the low-power networks. This inverter can hardly be used in high-power furnaces and those with a low arc voltage.
The use of a circuit with a controlled reactor makes it possible to reduce the number of control steps of the transformer, because the inductive reactance of the reactor can be controlled. However, this is attained by complicating the reactor design and using a complex system to control the reactor. The remagnetization of the reactor rods in this system need relatively long time so that the compensation of fast current variation of the electric arc furnace is insufficient, and the voltage fluctuation in the network cannot be adequately reduced. In addition, such a power supply has a low power factor and a high level of the higher harmonics of the consumed current.
Therefore, the above-mentioned technical solutions have not found wide practical application.
Known in the art is a controlled power supply for an electric arc furnace based on a thyristor rectifier.
The power supply comprises a controlled ac-to-dc inverter and a device for controlling the furnace arc. The inverter has a step-by-step controlled transformer whose secondary winding is connected to a thyristor bridge which supplies dc current to the furnace electrodes through a high-power smoothing choke.
The device for controlling the arc current has a circuit to control the arc current with current regulator whose input is fed with a difference between a preset reference value and the measured actual value of the dc arc current. The output signal of the current regulator is summed up with the measured actual value of the arc voltage, which is used as a preset current value and applied to the thyristor rectifier. The arc control device also has a voltage control circuit with a unit to control the electrode position to set a gap between the electrode and the smelting bath, and a voltage regulator whose input is fed with a difference of the preset value of the arc voltage and the measured value. The output signal of the voltage regulator is applied to the input of the device to control the electrode position. The circuits to control the current and voltage and a pulse shaper to send control pulses to the thyristors form a complex control system in the power supply. Such a power supply, as a whole, is complicated due to the presence of a special system to control the thyristors, features a heavy weight and large dimensions, as well as high installed power of the power supply equipment.
The power supply equipped with a thyristor rectifier allows one to continuously control the rectifier current what is necessary, on the one hand, for controlling the arc power to meet the technological requirements and, on the other hand, to limit the arc current fluctuation including the operating short-circuit current. However, owing to slow response, the thyristor rectifier cannot change the input voltage following the fast variations of the arc voltage so that a high-power smoothing choke is needed.
In order to avoid frequent arc breaks, the rated arc voltage must be by at least 20-25% lower the no-load voltage of the controlled rectifier. This fact leads to a corresponding increase of the installed power of the transformer and a decrease of the nominal power factor to 0.7-0.8.
The thyristor rectifier distorts the form of the current consumed from the power supply network and this leads to inadmissible distortion of the network voltage curve and to disturbance in the operation of the other consumers of this network in the case of insufficient short-circuit power of the supply network.
To compensate the higher harmonics of the current and reactive power, use is made of a filter and compensator unit. If the controlled rectifier does not allow one to adequately compensate the voltage fluctuation in the given network, it is necessary to perform dynamic compensation of the reactive component of the current and this complicates the power supply system.
Known in the art is a three-phase power supply for a single electrode smelting furnace comprising a transformer whose primary windings are connected to the power line, a controlled thyristor rectifier connected to the secondary windings of the transformer and based on a bridge circuit, and a reactor in the dc circuit connected to one output of the thyristor rectifier. The smelting furnace electrode is connected to the other output of the thyristor rectifier.
The comparatively high reactive component in the consumed current, whose value follows the power network voltage fluctuation, does not allow one to adequately reduce this fluctuation.